Latices of polychloroprene are industrially produced by free-radical emulsion polymerization of chloroprene (2-chloro-1,3-butadiene), if appropriate by copolymerization with suitable monomers. Such latices will herein also be collectively referred to as “polychloroprene latices”. The monomers are admixed in an aqueous medium with an emulsifier system in the course of latex production. This emulsifier system is generally anionic in nature, but occasionally nonionic or cationic systems are also used. The temperature range in which the polymerization is carried out comprises values of about 0° C. to about 60° C. The polymerization can thus be initiated by thermally disintegrating free-radical formers, or by means of redox systems. Generally, molecular weight regulators such as mercaptans or xanthogen disulphides are also used. In some cases, the molecular weight of the end product is also set by copolymerizing with sulphur and then splitting the resulting sulphidic bonds. The desired degree of conversion is set by stopping the reaction with a suitable agent. In the overwhelming number of cases, the resulting dispersion of polychloroprene in water, i.e. the so-called polychloroprene latex, is subsequently demonomerized by passing steam through it. A portion of the product obtained as a result finds direct industrial use as a latex, but the larger portion is freed of adherent water by coagulation, usually freeze coagulation, and shipped as a solid product to its ultimate use.
The polymerization generally by-produces products such as, for example, dimers of chloroprene or other low molecular weight compounds which can have an undesirable influence on the odour of the final product, whether it is to be used as latex or else as a solid rubber. These by-products are only imperfectly removable by the above-described steam treatments in the course of latex devolatilization or else by water wash in the course of the work-up of the solid rubber.
One way to reduce the undesirable odorant by-products to some degree at least consists in conducting the polymerization at a reduced polymerization temperature. However, a reduced polymerization temperature means directly a reduced polymerization rate and therefore is undesirable for economic reasons. It is further known that the crystallization tendency of polychloroprene products more often militates against the planned use.
One technologically important property of synthetic latices is the so-called gel content, which characterizes the fraction of the polymer that is insoluble in a certain solvent (toluene, for example). A high gel content is indispensable in various applications, for example for a high strength of a “wet”, i.e. incompletely dried, film of the respective material. A high gel content is achievable when using chloroprene (as in the case of all emulsion-polymerized dienes) by crosslinking by means of multifunctional monomers or by a high conversion for the polymerization reaction. A high conversion (>95% for example), however, is associated with economically less desirable long polymerization times, in particular when relatively low polymerization temperatures are employed.
Yet an economically better polymerization at relatively high temperatures, which leads faster to the desired degrees of conversion, results in increased formation of dimeric by-products via Diels-Alder reactions, and there are other secondary reactions observed to take place. This frequently gives rise to products which lack utility for some applications due to the odour nuisance generated during production or use of the end products.
Various methods have been described in the past of “masking” the odorant by-products by adding scents having a more intense odour. Examples of such masking agents are “scent of hay”, anisaldehyde, vanillin, camphor. This method is described by J. C. Carl in “Neoprene Latex”, E.I. DuPont de Nemours (self-published monograph), 1962, page 67. WO-A-01/00683 describes a method comprising the removal of odorants by means of ozone or hydrogen peroxide. Common to all these methods is that they mask or remove the odorants formed, but do not avoid or minimize their formation.